The present invention relates to a combine harvester for cereals, and relates more particularly to a device for primary separation of the grain from the straw, stalks, or other wastes, after they have been through the threshing device.
In most machines that are commercially available at present, the grains are separated from the straw downstream from the threshing device by means of perforated oscillating shakers over which the threshed grain and straw move rearwards relative to the longitudinal direction of the machine with the grains falling progressively through the straw down to the perforated plates at the bottoms of the shakers, through which they pass to be taken up in devices for final separation from small pieces of straw and other dust, e.g. by blowing away the chaff.
For economic reasons, these machines are becoming larger and larger in size and more and more powerful in order to process larger volumes of cereals in as short a time as possible.
However, when the machine is travelling over sloping ground, the effectiveness of the shakers is reduced.
It is also observed that the bed of straw moving over the shakers sometimes becomes several tens of centimeters thick such that grains fall relatively slowly through such a thickness and the shakers need to be extended considerably.
Other solutions have also been developed, likewise based on vibrating trays or the like, generally by increasing the number of threshing stages, but consequently also increasing the complexity and the cost of the machine, as well as the power it consumes, not to mention the danger of the machine becoming completely jammed when the straw clogs the threshing stages, which sometimes happens under certain harvesting conditions.
Another solution, illustrated in French patents 1 006 384, 2 030 564 and 2,382,847, and U.S. Pat. No. 1,907,344 consists in causing the grain-and-straw mixture to circulate after threshing in a rotating drum having a perforated wall and extending along the longitudinal axis of the machine.
The advantage of this solution is that it makes this separation stage insensitive to slope transverse to the direction of travel of the machine across sloping ground.
In the separator disclosed in FR patent 1 006 384 the crop material advances under gravity, thanks to an adequate slanting of the drum axis.
The grain and straw mixture is entrained by a helical rib disposed around the inside periphery of the drum. However, this gives rise to relatively slow drive and this type of solution has not been developed in practice since the size of drum needed would be excessive, and/or since it would be necessary to add complementary driving means, such as a blower as disclosed in this document.
The separator disclosed in FR patent 2 382 847, comprises, in addition to a helical rib attached to the inner periphery of the drum, a mechanism extending along the axis of the drum and provided with reciprocating arms intended to drive longitudinally the crop materials. This mechanism is however complex, expensive and fragile.
In the separator disclosed in U.S. Pat. No. 1,907,344, the inner face of the drum carries a great number of inwardly and rearwardly extending rods. Due to this great number, the whole set of rods actually forms a generally cylindrical surface having a shallow helicoidal relief having a very low pitch.
The crop materials are not efficiently driven and it is necessary to add a complementary blowing means in order to correctly drive the crop material. Further, the grains, chaff and other small size particles are virtually not driven along the drum.